The present invention relates to plasma-arc machining of electrically conductive materials and more particularly to methods and apparatus for plasma-arc machining of materials. The method and apparatus of the present invention may find application in various branches of industry concerned with the manufacture of welded and clad structures.
In the present-day practice of manufacturing welded structures, particularly from aluminium and its alloys, shielded and submerged DC and AC arc welding with a consumable and nonconsumable electrode is employed.
A general disadvantage of the AC arc welding with a nonconsumable electrode are the limited technological possibilities of such a procedure as to the thickness of parts that can be welded together without veeing (V-shaped configuration). An attempt to remedy these limitations by resorting to heavy-current AC arcs leads to an increase of tungsten inclusions, oxide films and porosity in the weld metal.
A disadvantage of shielded arc welding with a consumable electrode are the limited technological possibilities offered by single-pass one-side welding, while submerged arc welding with a consumable electrode is disadvantageous in view of poor sanitary working conditions. Welds made by the shielded arc techniques with the use of a consumable electrode are characterized by the presence of pores and oxide inclusions, while welds made by the submerged arc procedure and with the use of a consumable electrode display slag inclusions and low stability against corrosion. Moreover, this method of welding is associated with an extremely high consumption of welding wire.
The method of DC reverse polarity plasma-arc welding, despite its numerous advantages, so far has found little industrial application.
There is already known in the art a method of DC reverse polarity plasma-arc welding of aluminium sheets having a thickness of 250" (0.635 cm) (see, e.g. Cathodic Cleaning and Plasma-Arc Welding of Aluminum. Welding Journal No. 5, 1968).
This known method of DC reverse polarity plasma-arc welding was used on equipment designed for plasma cutting and sputtering, depending on the configuration of the nozzle-electrode. The set of equipment, besides a modified 50 kW universal plasma torch, also included monitoring equipment adapted to control voltage and current intensity of the arc, as well as the flow of plasm-forming and shielding gases. The plasma arc was powered by two series-connected DC rectifiers with a no-load voltage of 140 to 150 V.
For a better dissipation of heat, a copper water-cooled electrode was used instead of a tungsten one.
The shielding and plasma-forming gas was either argon or a mixture of argon and helium.
The attained rates of plasma arc-welding were equal to or somewhat exceeded those attainable with other methods of welding. Maximum thickness of aluminium welded by DC reverse polarity plasma-arc welding per pass without veeing was 0.250" (0.635 cm).
Further developments of the method of DC reverse polarity plasma-arc welding are reported (see, e.g. the publication "Plasma-Arc Welding of Aluminum" in "Keikinzoku Yosetsu", 9, No. 98, 1971); this method was employed for welding aluminuim sheets whose maximum thickness was 6 mm.
In this known method, the electrode of the plasmatron is coupled to the positive terminal of the DC power supply source with a view to ensuring heat dissipation, the positive electrode in the plasmatron is made of copper, with the end face in its working portion shaped as a frustum of a cone (the diameter of the top surface area being about 2 mm).
The process of DC reverse polarity plasma-arc welding is working as follows. Simultaneously with feeding working and shielding gases and water for cooling the positive electrode and the nozzle, a high-frequency current discharge is passed to initiate the pilot arc. Then a flash of plasma emitting from the nozzle is produced by the pilot arc, and a main plasma-arc is ignited between the positive copper electrode and the plate to be machined, provided with output straps. The pilot arc is quenched, the plasmatron is moved to the required position, and welding is started.
The above-considered methods of DC reverse polarity plasma-arc welding are not free from a number of essential disadvantages: the main disadvantages are as follows:
a. noncompetitiveness with other types of arc welding; PA1 b. lower permissible values of working currents; PA1 c. restricted ranges or permissible working conditions; PA1 d. limitations to the thickness of welded sheets; PA1 e. necessity of using power supply sources with a no-load voltage of 140 to 150 V while welding sheets having a thickness from 0.6 to 6.0 mm, the intensity of working currents reaching 200 A; PA1 f. possibility of the weld metal being contaminated with the anode material (copper).
At present, clad structures are manufactured through the use of arc processes, namely, by surfacing with a strip electrode, by shielded and submerged surfacing with a consumable electrode, as well as by plasma-arc surfacing with a solid wire electrode, with a wire tube electrode, and with powders.
A disadvantage common to and characteristic of all the methods of arc surfacing is a high degree of melting of the back-up material. The methods of surfacing are noted for their sufficiently high efficiency; however, the required degree of melting can be ensured only if the thickness of the built-up layer is much greater than that needed in the finished product. The necessity of subsequent finishing, naturally, brings down the efficiency of the surfacing procedure in terms of the product.
Various known methods of plasma-arc surfacing (see, e.g. Plasma-Arc-Hot-Wire Surfacing - A New High Deposition Process Welding Journal No. 5, 1969 and Plasmennaya naplavka metallov (Plasma-arc Surfacing of Metals), "Mashinostroyeniye", Leningrad, 1969) fail to eliminate melting of the back-up material.
With the use of the known plasma-arc surfacing methods on an industrial scale, the degree of melting reaches 5 to 10 percent, depending on the current magnitude and the built-up material.
It is an object of the present invention to eliminate the above-mentioned disadvantages.
Another object of the invention is to provide a method of DC reverse polarity plasma-arc machining which will include plasma-arc welding and plasma-arc surfacing.
A further object of the invention is to provide apparatus for effecting DC reverse polarity plasma-arc machining.
Still another object of the invention is to provide universal equipment of the type described.